Effect of Sulfur Oxidation on the Transmission Mechanism of

J. Phys. Chem. A 2009, 113, 2647–2651

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Effect of Sulfur Oxidation on the Transmission Mechanism of 4JHH NMR Coupling Constants in 1,3-Dithiane Gisele F. Gauze,† Ernani A. Basso,† Rube´n H. Contreras,‡ and Cla´udio F. Tormena*,§ Departamento de Quı´mica, UniVersidade Estadual de Maringa´, CEP-87020-900, Maringa´, PR, Brazil, Department of Physics, FCEyN, UniVersity of Buenos Aires and CONICET, Buenos Aires, Argentina, and Department of Organic Chemistry, Chemistry Institute, P.O. Box 6154, UniVersity of Campinas, UNICAMP, CEP: 13084-971 Campinas, SP, Brazil ReceiVed: December 12, 2008; ReVised Manuscript ReceiVed: January 15, 2009

Long-range 4JHH couplings in 1,3-dithiane derivatives are rationalized in terms of the effects of hyperconjugative interactions involving the SdO group. Theoretical and experimental studies of 4JHH couplings were carried out in 1,3-dithiane-1-oxide (2), cis-1,3-dithiane-1,3-dioxide (3), 1,3-dithiane-1,1,3-trioxide (4), and 1,3-dithiane1,1,3,3-tetraoxide (5) compounds. Hyperconjugative interactions were studied with the natural bond orbital, NBO, method. Hyperconjugative interactions involving the LPO, oxygen lone pair and σ*C2-S1 and σ*S1-C6 antibonding orbitals yield an increase of 4JHeq-Heq couplings. Long-range 4JHax-Hax couplings were also observed between hydrogen atoms in axial orientation, which are rationalized as originating in hyperconjugative interactions involving the bonding σC6-Hax and antibonding σ*SdO orbitals. The symmetry for orbital interactions is possible only when the SdO group is in the axial orientation. Introduction Stereochemical properties of NMR spin-spin coupling constants (SSCCs) are mostly based on the ability of their Fermi contact (FC) terms for being transmitted through the electronic molecular structure.1,2 For nJXY couplings with n e 3 as well as for through-space SSCCs, exchange interactions play a key role in their transmission. On the other hand, for n > 3, chargetransfer interactions are excellent carriers for the spin information associated to the FC interaction.3 In this context, the most important sources of stereochemical information were, for many years, 3JHH SSCCs, which are strongly dependent on the torsion angle of the respective vicinal hydrogen atoms, as shown by Karplus.4,5 During the past few years, there was not only an important increase in the understanding of that transmission but also there was a large increase in the possibility for calculating SSCCs for medium- to large-sized molecules and, therefore, during the past decade other types of SSCCs like, for instance, 1 JCH,6-8 and 2JCH9 besides 3JCH10-15 SSCCs were considered for stereochemical purposes. Whereas the 3JHH couplings are widely applied to assign conformational preferences,16-20 the same attention is not paid to long-range 4JHH SSCCs, probably due to its usually smaller absolute values than those of 3JHH SSCCs. However, a few interesting 4JHH SSCCs are worth mentioning like those in fused cyclopropanes,21 which resemble the behavior of those in allylic systems studied by Garbisch et al.22 It should be mentioned that to get accurate values for 4JHH SSCCs the 1 H NMR spectra should be acquired with high digital resolution. In a recent paper, Dias and co-workers23 discussed the potential of 4JHH SSCCs to determine conformational preferences by studying the conformational equilibrium in alkyl and silyl ethers, which were determined mainly by intramolecular hydrogenbonds. * To whom correspondence should be addressed. [email protected]. † Universidade Estadual de Maringa´. ‡ University of Buenos Aires and CONICET. § University of Campinas.

Figure 1. Structures for studied compounds: 1,3-dithiane (1), 1,3dithiane-1-oxide (2), cis-1,3-dithiane-1,3-dioxide (3), 1,3-dithiane-1,1,3trioxide (4), and 1,3-dithiane-1,1,3,3-tetraoxide (5).

To get deeper insight into factors affecting the efficiency of JHH coupling pathways, the 1H spectra of several 1,3-dithiane compounds (Figure 1) with different degrees of sulfur oxidation are studied in the present work. Hyperconjugative interactions along the 4JHH coupling pathway involving both bond and antibonding orbitals were analyzed using the NBO24 approach, to rationalize the influence of electronic delocalization interactions on transmission mechanisms of 4JHH coupling in oxidized 1,3-dithianes (Figure 1). Sulfur-containing six-membered rings and its sulfoxide and sulfone derivatives have been widely studied.25 The enthalpies of formation and the energy of the SdO bond for several sixmembered ring sulfoxides and sulfones were determined26-30 including also conformational analyses13-15,31,32 and stereoelectronic effects on nucleophilic addition to alkylidene sulfoxide.33 In the NMR case, some studies were performed to analyze the stereoelectronic effects on 1JCH coupling constants of sulfur lone pairs8,34-36 and of SdO bonds corresponding to sulfoxide and sulfone37 groups. However, there are not analyses about stereoelectronic effects on long-range SSCCs, such as 4JHH, which can be used to determine not only molecular structures but also conformational preferences.23,25 4

10.1021/jp810981z CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

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Gauze et al. TABLE 1: Experimental and Calculateda Long-Range 4JHH SSCCs (in Hz) for 1-5 4

J

1

2

3

4

5b

H2axH6ax H2axH4ax H2eqH6eq H2eqH4eq

(0.87) (0.87) (1.19) (1.19)

0 (0.31) 0 (0.24) 2.60 (2.45) 0 (0.80)

0 (0.36) 0 (0.36) 2.02 (2.23) 2.02 (2.23)

1.51 (1.10) 0 (0.32) 2.11 (2.77) 2.11 (2.06)

1.07 (0.98) 1.07 (0.98) 1.07 (2.23) 1.07 (2.23)

a Calculated values are in parentheses. b Experimental SSCCs are the average value at room temperature.

Figure 2. Bicyclo[1.1.1]pentane (6) and norbornanone (7). 38

4

It is well-known that JH1H5 SSCCs are transmitted with enough efficiency that allows their experimental observation when the intermediate bonds H1-C2-C3-C4-H5 are in a planar W arrangement. In such a case, charge transfer interactions defining its coupling pathway are mainly σC3-C4 f σ*C2-H1 and σC2-C3 f σ*C2-H139 and their respective back-donation interactions. A very efficient 4JH1H3 ) 18.2 Hz SSCC was reported in bicyclo[1.1.1]pentane, 6 (Figure 2).40 It is noted that the H1 and H3 protons are connected by three planar equivalent W coupling pathways, where the corresponding C-C bonds are very strained and, therefore, are very good electron donors by σ-hyperconjugative interactions, facilitating the FC transmission between H1 to H3. However, it should also be recalled that a throughspace contribution between the C1 and C3 bridge-head carbon atoms might be transmitted41 and that both coupling protons are in a configuration that favors a non-negligible and negative diamagnetic spin orbit, DSO, contribution.42 Parella et al.43 reported in norbornanone (7) (Figure 2), 3,4JC1,H4 ) + 5.2 Hz and 3,4JC4,H1 ) + 8.5 Hz. This unexpected asymmetry was recently rationalized44 as originating in an additional coupling pathway activated in the four-bond contribution through the C1-C2-C3-C4 bridge containing the carbonyl moiety. This additional coupling pathway takes place through the simultaneous occurrence of these two hyperconjugative interactions, σC1-C7 f σ*C2dO and σC3-C4 f σ*C2dO, which enhances the ability of the H1-C1-C2-C3-C4 four-bond coupling pathway for transmitting the FC term of 3,4JC4,H1 SSCC. It is highlighted the similitude of this additional coupling pathway to that of homoallylic couplings, which have been known since many years ago,45,22 the main difference being this, whereas in homoallylic couplings the intermediate antibonding orbital is of π* symmetry, in 3,4JC4,H1 ) + 8.5 Hz, 7, is of σ* symmetry. On similar grounds can be rationalized the differences observed in the same compound,46 (Figure 2) for 4JH3nH7a ) 4.2 Hz and 4JH6nH7s ) 2.3 Hz. For the former SSCC strong interactions of types σC3-Hn f π*C2dO and σC1-C7 f π*C2dO should constitute an additional coupling pathway, rendering 4 JH3nH7a > 4JH6nH7s. However, it should be highlighted that in this case, the intermediate antibonding orbital defining this coupling pathway is of π* symmetry, like in homoallylic SSCCs. Computational Details All structures were fully optimized at the B3LYP functional47 and aug-cc-pVDZ basis set48 level using the Gaussian 0349 program. Electronic structures from 1-5 were studied using NBO analysis50 at the B3LYP/cc-pVDZ level using the geometries optimized. The 4JHH couplings in 1-5 were also calculated using the CP-DFT/B3LYP methodology51 as implemented in the Gaussian 03 package49 of programs. All four terms of 4JHH spin-spin coupling constant (Fermi contact, FC; spin dipolar, SD; paramagnetic spin orbit, PSO; and diamagnetic spin orbit, DSO)

were carried out using the EPR-III basis set,52 which is a triple-ζ type and includes diffuse and polarization functions. The s part of this basis set is enhanced to better reproduce the electronic density in the nuclear regions because this point is particularly important when calculating the FC term.53 The EPR-III basis set was used for the carbon and hydrogen atoms and cc-pVDZ basis set for other atoms present in the molecules studied. Experimental Section NMR Experiments. 1H NMR spectra were recorded on spectrometer operating at 300.25 MHz for 1H. Measurements were carried out at a probe temperature of the 25 °C, using solutions of ca. 10 mg cm3 in CDCl3 for 2 and in DMF-d7 for 3-5. The 1H spectra were based on the TMS reference. Typical conditions for 1H spectra were: 16 transients, spectral width 2500 Hz, with 32 k data points, giving an acquisition time of 6.35 s and zero filled to 128 k to give a digital resolution of 0.04 Hz/ point. Compounds. 1,3-dithiane (1) and 1,3-dithiane-1,1,3,3-tetraoxide (5) are commercially available and were used without further purification, whereas 2-4 were prepared according procedure described in the literature.54 Results and Discussion The experimental 4JHH coupling constants for studied compounds (Figure 1) were measured directly from the 1H NMR spectra, whereas theoretical values were calculated at the B3LYP/EPR-III level of theory (Table 1). The experimental trends of 4JHH SSCCs are correctly described by theoretical calculations, as observed in Table 1. Longrange 4JHeqHeq can only be observed when the sulfur atom in the coupling pathway is oxidized to sulfoxides 2 and 3 with SdO group at equatorial position. For sulfones 4 and 5, it can also be observed long-range 4JHaxHax SSCCs between hydrogen atoms in the axial position, suggesting that they are observed due to the axial orientation of the SdO group. The effect of sulfur oxidation and orientation of the SdO bond on the 4JHH coupling pathway can be easily observed directly from 1H NMR spectra for 2-5, shown in Figure 3, where for the sake of simplicity only signals for hydrogen atoms (axial and equatorial) at position 2 are displayed. In 2, as shown in part a of Figure 3, for both protons (H6eq and H4eq) in the equatorial position, only 4JH2eqH6eq is observed, whereas 4JH2eqH4eq cannot be detected. It is noted that the main difference between the corresponding coupling pathways is that in the former the sulfur atom is oxidized (sulfoxide), whereas in the latter the corresponding sulfur atom is not oxidized. The relevant role played by the sulfur oxidation along the 4JHeq-Heq four-bond coupling pathway is also appreciated when analyzing the 1H NMR spectra for 3 and 4. It is noted that in 3 the SdO groups are in equatorial position (this is the only stable form55-57 for this compound) and was observed a doublet of triplets for H2eq signal (part b of Figure 3) due to the geminal coupling

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Figure 3. 1H NMR signals for H2 axial (low frequency) and equatorial (high frequency) for: (a) 1,3-dithiane-1-oxide (2); (b) cis-1,3-dithiane1,3-dioxide (3); (c) 1,3-dithiane-1,1,3-trioxide (4); and (d) 1,3-dithiane-1,1,3,3-tetraoxide (5).

with H2ax and the two four-bond couplings with H4eq and H6ax. A similar pattern is observed for H2eq in 4, however in this case a long-range coupling, 4JH2ax,H6ax is also observed for H2ax, which suggests that the presence of the SdO moiety in axial orientation enhances the ability of this coupling pathway for transmitting that long-range SSCC. To get insight into the influence of an SdO group and its orientation for enhancing the four-bond JHH coupling pathways in dithiane derivatives measured in this work, hyperconjugative interactions involving either bond and/or antibonding orbitals belonging to those pathways are studied performing the relevant NBO calculations. 4 JH2eq-H6eq SSCCs could be measured in 2-5. It is observed that in 5 this SSCC is less than one-half those measured in 2-4. On the other hand, 4JH2eq-H4eq could be measured in 3-5. Again, in 5 this SSCC is less than one-half of those measured in 3 and 4. To verify if H2eq-C2-S1-C6-H6eq and H2eq-C2-S3-C4H4eq coupling pathways are enhanced when either or both S1 and S3 are oxidized, in Table 2 are listed hyperconjugative interactions within both pathways. In this Table is shown the sum of each hyperconjugative interaction and its respective back-donation because they are considered to be adequate descriptors to compare how efficient are the respective coupling pathways. Data in Table 2 show similar hyperconjugative interactions along the 4JHeq-Heq coupling pathways for 1-5, the result of which is not in line with experimental 4JHeq-Heq SSCCs displayed in Table 1. Therefore, a different type of coupling pathway enhancement should be sought. For instance, in 2 the 4JH2eq-H6eq coupling pathway is notably enhanced in comparison with that of 4JH2eq,H4eq, which could not

TABLE 2: Hyperconjugative Interactions within the H2eq-C2-S1-C6-H6eq and H2eq-C2-S3-C4-H4eq Coupling Pathways of 4JH2eq,H6eq and 4JH2eq,H4eq, Respectively; in This Table Each Entry Corresponds to the Sum of the Interaction Shown at Left and Its Respective Back-Donation Interaction (in kcal/mol) interactions 1 2 3 4

σC2-Heq f σC6-Heq f σC2-Heq f σC4-Heq f

σ*C6-S1 σ*S1-C2 σ*C4-S3 σ*S3-C2

1

2

3

4

5

3.2 3.3 3.2 3.3

2.9 2.8 3.4 3.5

2.9 3.2 2.9 3.2

3.1 3.4 2.8 3.2

2.8 3.4 2.8 3.4

be measured. This suggests that the strong hyperconjugative interactions (LP(O1eq) f σ*C2-S1 ) 22.6 kcal/mol and LP(O1eq) f σ*C6-S1 ) 20.1 kcal/mol) that take place within the sulfinyl group enhances the 4JH2eq,H6eq coupling pathway in 2, 4JHeq,Heq SSCCs measured in 3 seem to support this assumption, although both 4JHeq,Heq SSCCs are smaller than in 2. In 4, experimental 4 JH2eq,H6eq and 4JH2eq,H4eq SSCCs are equal, although the coupling pathway in the former bears two sulfinyl group. For similar coupling pathways in 5, both of 4JH2eq,H6eq and 4JH2eq,H4eq SSCCs are notably smaller, suggesting that the second sulfinyl group with an axial orientation yields an inhibition of that coupling pathway. No measurement could be carried out to verify the efficiency of a coupling pathway bearing only a sulfinyl group in the axial position. The rationalization of the transmission of 4JHax,Hax SSCCs in 4 and 5 is rather straightforward. It must be highlighted that these coupling pathways are quite similar to those of homoallylic couplings.45 The difference rests on this fact, whereas in homoallylic couplings the intermediate antibonding orbital is of π* symmetry; in 4 and 5 they are of σ* symmetry. Such

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TABLE 3: Interactions (in kcal/mol) that Define the Coupling Pathway Transmission of 4JHax,Hax SSCCs in 4 and 5 A B C D

interactionsa

4

5

σC2-Hax f σ*S1dOax σC6-Hax f σ*S1dOax σC2-Hax f σ*S3dOax σC4-Hax f σ*S3dOax

2.8 3.0

2.6 3.0 2.6 3.0

a Back-donation interactions are summed to interactions quoted in the second column.

interactions are displayed in Table 3, where it is noted that interactions A and B define the coupling pathway for 4JH2ax,H6ax, whereas interactions C and D define the coupling pathway for 4 JH2ax,H4ax. Conclusions In the present study, rationalizations for the transmission mechanisms involving the long-range 4JHH coupling constant for 1,3-dithiane derivatives are presented. It was observed that these long-range couplings are very sensitive to the sulfur oxidation because the 4JHeq-Heq coupling transmission mechanism involves interactions between LPOfσC-S, which is present only when the respective sulfur atom is oxidized. Besides, 4JHH coupling constants are also sensitive to the orientation of SdO bond because the 4JHax-Hax coupling transmission mechanism involves the hyperconjugative interactions of types σC4-H4ax f σ*SdO, σC6-H6ax f σ*SdO and σC2-H2ax f σ*SdO, which take place only when the SdO bond is in axial orientation. Therefore, longrange 4JHH coupling constants can be used to determine the orientation of SdO bonds in sulfur derivatives compounds, which are difficult tasks in structural assignments. Acknowledgment. The authors are grateful to FAPESP (grant 06/03980-2, 05/59649-0) for the financial support of this work. To CNPq for financial support and for fellowships (to EAB and C.F.T) and CAPES to scholarship (to G.F.G.). Financial support from CONICET (PIP 5119/05) and UBATEC (X047) to RHC are gratefully acknowledged. References and Notes (1) Soncini, A.; Lazzeretti, P. J. Chem. Phys. 2003, 119, 1343. (2) Castillo, N.; Matta, C. F.; Boyd, R. J. J. Chem. Inf. Model. 2005, 45, 354. (3) Krivdin, L. B.; Contreras, R. H. Annu. Rep. NMR Spectrosc. 2007, 61, 133. (4) Karplus, M. J. Chem. Phys. 1959, 30, 11. (5) Karplus, M. J. Am. Chem. Soc. 1963, 85, 2870. (6) Juaristi, E.; Cuevas, G. Acc. Chem. Res. 2007, 40, 961. (7) Contreras, R. H.; Esteban, A. L.; Diez, E.; Della, E. W.; Lochert, I. J.; dos Santos, F. P.; Tormena, C. F. J. Phys. Chem. A 2006, 110, 4266. (8) Kleinpeter, E.; Koch, A.; Pihlaja, K. Tetrahedron 2005, 61, 7349. (9) Perez, C.; Suardı´az, R.; Ortiz, P. J.; Crespo-Otero, R.; Bonetto, G. M.; Gavı´n, J. A.; Garcı´a de la Vega, J. M.; San Fabian, J.; Contreras, R. H. Magn. Reson. Chem. 2008, 46, 846. (10) dos Santos, F. P.; Tormena, C. F.; Contreras, R. H.; Rittner, R.; Magalha˜es., A. Magn. Reson. Chem. 2008, 46, 107. (11) Aydin, R.; Gu¨nther, H. Magn. Reson. Chem. 1990, 28, 448. (12) Pedersoli, S.; dos Santos, F. P.; Rittner, R.; Contreras, R. H.; Tormena, C. F. Magn. Reson. Chem. 2008, 46, 202. (13) To´th, G.; Kova´cs, J.; Le´vai, A.; Kleinpeter, E.; Koch, A. Magn. Reson. Chem. 2001, 39, 251. (14) Kova´cs, J.; To´th, G.; Simon, A.; Le´vai, A.; Koch, A.; Kleinpeter, E. Magn. Reson. Chem. 2003, 41, 193. (15) Heydenreich, M.; Koch, A.; Kova´cs, J.; To´th, G.; Kleinpeter, E. Magn. Reson. Chem. 2004, 42, 667. (16) Anizelli, P. R.; Vilcachagua, J. D.; Cunha Neto, A.; Tormena, C. F. J. Phys. Chem. A 2008, 112, 8785. (17) Cedran, J. C.; dos Santos, F. P.; Basso, E. A.; Tormena, C. F. J. Phys. Chem. A 2007, 111, 11701.

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